I would suggest "use with caution" the "special character tag" approach. A couple of reasons. First you might find that in a future version of search your tag characters conflict with some special search syntax, so then you have to change them and "unlearn" what you may have adapted yourself too.
But far more importantly also are you 100% sure that you are the only person who will ever browse your library in future? No intention of a family member or friend sitting down to browse it or shared with them over the web? Because you are going to find it incredibly painful to explain to them that "[!@$sf" is your foo genre or whatever.
My point being - again I don't think it should be a "recommended" approach in your guide. For sure you can mention it as something that you like, but in my opinion it is not something that new users should be steered towards.
Now if you think I'm totally anti using symbols as tags, you would be wrong. As I have mentioned previously I did use to use a number of them, but right now I am down to just two:
a minus sign - for a wishlist item that I put on my empty books.
an asterisk * for a book that I need a better quality format for.
Sorry unboggling, I just can't grok the prefixing of % signs, brackets and all other sorts of things to genres or whatever. I think you are out on a limb with that one - I understand that you like it for yourself but I would hate to have to try to explain it to someone else
